Solar energy is energy derived ultimately from the Sun. The amount of solar radiation that reaches the Earth is only about one billionth of the total energy that the Sun generates, or the equivalent of about 420 trillion kilowatt-hours. The amount of energy that strikes the Earth's surface in one day exceeds daily consumption by a factor of 10,000 to 15,000.
Besides passive solar design, i.e using different methods of construction to take advantage of the Sun (see solar architecture), solar radiation can also be used actively. Solar cells produce "clean" electric current ready for use, whereas a solar heating system transforms the radiation into heat.
Solar energy can be divided into direct and indirect categories. Most energy sources on Earth are forms of indirect solar energy, although we usually don't think of them in that way. Coal, oil, and natural gas derive from ancient biological material that took its energy from the Sun (via photosynthesis) millions of years ago. All the energy in wood and foodstuffs also comes from the Sun. Movement of the wind (which causes waves at sea), and the evaporation of water to form rainfall, which accumulates in rivers and lakes, are also powered by the Sun. Therefore, hydroelectric power and wind and wave power are forms of indirect solar energy. Direct solar energy is what we usually mean when we speak of solar power – it is the use of sunlight for heating or generating electricity.